1. Field of the Invention
The present invention relates generally to insulated-gate field-effect transistors and more particularly to such transistors formed on a single substrate and having different gate capacitances.
2. Description of the Related Art
In some circumstances, for example, static random access memory (SRAM) implemented with insulated-gate field-effect transistors (IGFETs), it is desirable to form IGFETs on a single semiconductor substrate with some having substantially larger gate capacitances than others. An SRAM cell implemented with IGFETs typically includes a bistable circuit in the form of a cross-coupled flip-flop. A pair of gating transistors couple storage transistors in the cell to different bit lines. The gating transistors are turned on and off to read the state of the cell, i.e., to apply a high or low voltage to the associated bit line. When one of the gating transistors is turned on, charge flows from the storage transistor and, if sufficient flow occurs, thereby change the state of the cell. It is obviously undesirable for the cell state to change when the state is read. To prevent such unintended state changes, the storage FET in the cell is typically made 4-5 times larger than that of the gating FET. A storage transistor having a substantially larger area, and therefore a substantially larger capacitance, prevents substantial charge flow during a read operation thereby preventing an unintended state change in the cell responsive to the read operation.
Utilizing transistors which are 4-5 times larger than other transistors on the substrate is disadvantageous, however, because it takes up area on the semiconductor substrate and thereby limits the number of devices which can be formed on the substrate.
It would be desirable to provide IGFETs formed on a single substrate and having different gate capacitances which utilize less area than such prior art IGFETs.